1. Field of the Invention
The present invention relates to an image forming device for obtaining a multicolor image by superimposing and transferring toner images, which have different colors from each other and formed on respective surfaces of a plurality of latent image carriers, onto a surface of an endless belt member or onto a recording member retained on the surface.
2. Description of the Related Art
The image forming device of this kind sometimes causes a color shift due to the transfer of each color toner image with positional displacements in a belt moving direction. One of the causes of the color shift is a relative shift between the latent image writing positions on each color latent image carrier. Specifically, if a latent-image-writing-related member such as a reflective mirror or a scan lens expands or contracts in response to temperature changes, the latent image writing positions are sometimes shifted relatively between each color latent image carrier. In case that such a shift of the latent image writing position occurs, a relative positional displacement of the latent image arises between each color latent image carrier, thereby color shift occurs.
Another cause of the positional displacement of each color toner image is a speed variance of the belt member due to an eccentricity of a driving roller for transferring a driving force to the belt member. Specifically, if the driving roller is eccentric, the belt member causes a speed variance with properties which draw a sine curve having one cycle per one revolution of the roller. Due to this speed variance, each color toner image is transferred with positional displacement from each color latent image carrier to the belt member or to the recording member on the surface of the belt member, thereby a color shift occurs.
Thus, the image forming device described in unexamined Japanese Patent Publication No. 2004-205717 reduces a relative shift of latent image writing positions between each color photoreceptor by periodically executing a writing position correcting process for correcting the latent image writing position relative to each color photoreceptor (i.e. each latent image carrier). Firstly, in the writing position correcting process, the image forming device transfers predetermined toner images formed on each color photoreceptor onto the surface of the belt member, and forms an image for detecting color shift on the surface of the belt member. Then, the image forming device calculates amounts of positional displacements of each toner image in a belt moving direction based on timing for detecting each color toner image formed on the color shift detecting image by reflective photo sensors. Next, based on the calculation results, the image forming device fine-adjusts an inclination angle of the reflective mirror in an optical scanning system for writing a latent image, or fine-adjusts timing for irradiating the photoreceptor. In this way, the image forming device can reduce color shift by reducing a relative shift of the latent image writing position between each color photoreceptor.
In addition, the image forming device stabilizes a speed of the belt member by executing a constant belt speed control for driving a driving motor so that the image forming device rotates the endless belt member at a constant speed based on detected result of a moving speed of the belt member. Specifically, the image forming device is provided with a rotary encoder attached to a driven roller which is one of a plurality of tensioning rollers for tensioning the belt member and which is rotated by the movement of the belt member. The image forming device detects a moving speed of the belt member based on the detection result of the rotary encoder. In the case where there is a speed variance of the belt member, the image forming device feedbacks the detection result of the rotary encoder to the driving motor to generate an opposite phase speed variance relative to the speed variance. In this way, the image forming device can reduce color shift due to the speed variance of the belt member by reducing the speed variance of the belt member due to the eccentricity of the driving roller and by stabilizing the speed of the belt member.
In this respect, the inventors have conducted an experiment for increasing the print speed by using a testing machine of the above mentioned image forming device and have found out that the testing machine is subject to a streaky image disturbance when using a cardboard as a recording paper. Specifically, the testing machine is configured to superimpose and primary-transfer color toner images onto the surface of the belt member, and to secondary-transfer the primary-transferred color toner images collectively from the belt member to a recording paper at a secondary transfer nip by bringing the belt member into contact with a secondary transferring roller. In such a configuration, if a cardboard is used as a recording paper, the testing machine instantaneously reduces the moving speed of the belt member significantly due to rapid increase in load, when feeding the cardboard into the secondary transfer nip. Under the condition of the print speed higher than before, the reduction rate also becomes larger. Consequently, if the testing machine feedbacks the speed reduction to the drive control of the driving motor, the testing machine instantaneously increases the speed of the belt member excessively. If such an instantaneous speed reduction at the time of feeding the cardboard into the nip and such a subsequent instantaneous speed increase occur, the testing machine causes the above mentioned streaky image disturbance without transferring toner images from the photoreceptors to the belt member properly. This streaky image disturbance is far more outstanding than the color shift caused by the eccentricity of the driving roller. Therefore, countermeasures should be taken in priority to the color shift.
The above mentioned testing machine is configured to superimpose and transfer each toner image of each color photoreceptor onto the belt member, and then secondary-transfer the toner images collectively to the recording paper at the secondary transfer nip. However, the following configuration can also cause a similar streaky image disturbance. The configuration superimposes and transfers each toner image of each color photoreceptor onto a recording paper retained on the surface of the belt member. This is because such a configuration causes instantaneous speed reduction and instantaneous speed increase of the belt member each time the testing machine feeds a cardboard into each color primary transfer nip by bringing each color photoreceptor into contact with the belt member.
To that end, the inventors are developing a novel image forming device for executing a constant motor speed control by using a FG signal when using a cardboard, instead of the above mentioned constant belt speed control. The FG signal is a signal sent from a FG signal generator (Frequency Generator) which generates a pulse wave each time it detects a predetermined rotational angle displacement of a motor shaft. In the constant motor speed control, the image forming device rotates a driving motor at a predetermined target rotational speed constantly by driving the driving motor to keep the frequency of the FG signal constant. As described above, when the cardboard enters into the nip, a speed of the belt member instantaneously decreases significantly. However, since the belt stretches at the same time, the rotational speed of the driving motor does not decrease that much. Thus, without detecting a rapid decrease of the rotational speed of the motor when a cardboard enters into a nip, the image forming device keeps the driving motor rotating stably at the target rotational speed from the entrance of the cardboard into the nip until the ejection of the cardboard from the nip. Consequently, the image forming device no longer instantaneously increases the speed of the belt member excessively just after the cardboard enters the nip. In such a configuration, although the image forming device does not prevent color shift caused by the eccentricity of the driving roller from occurring, the image forming device can reduce the above mentioned streaky image disturbance.
The inventors have produced a testing machine which switches the constant belt speed control to the constant motor speed control when using a cardboard, and have tested it. Then, the test resulted significant color shift. The inventors have found out that this significant color shift is caused by the following reason. As described above, the constant motor speed control rotates the driving motor at a predetermined target rotational speed. If a diameter of the driving roller is a value as planed, an average linear speed of the driving roller at the time becomes almost the same value as the predetermined target speed of the belt member. However, the driving roller is generally coated by material with large frictional resistance such as a rubber in order to exert a large grip force on the belt member. In such a driving roller, due to the limitation of the machining accuracy, an error in the diameter is unavoidable. In the driving roller including a slight error in the diameter, if the driving motor is rotated at the predetermined target rotational speed, the average linear speed of the surface of the driving roller slightly deviates from the target speed of the belt member. Due to this deviation, it is found out that the constant motor speed control has been moving the belt member at an average speed different from that in the constant belt speed control. Although the testing machine executes the above described writing position correcting process under the condition of the constant belt speed control, the testing machine can reduce color shift by the execution only when the testing machine drives the driving motor by using the constant belt speed control. This is due to the following reason. That is, once the testing machine switches the control method from the constant belt speed control to the constant motor speed control, the testing machine differentiates a subsequent average speed of the belt member from the average speed of the belt member during the writing position correcting process. Thus, the testing machine differentiates time, which is required for the belt member to move from an upstream primary transfer nip to a downstream primary transfer nip, from corresponding time during the writing position correcting process. Consequently, the testing machine can no longer superimpose toner images at each primary transfer nip without any displacement.
Although the testing machine is configured to execute the writing position correcting process under the constant belt speed control, if the testing machine executes the writing position correcting process under the constant motor speed control, the testing machine causes similar color shift when switching the control method of the driving motor from the constant motor speed control to the constant belt speed control.
In view of the above mentioned problems, it is an object of the present invention to provide a following image forming device. That is, the object is to provide an image forming device which can reduce the generation of a streaky image disturbance and which can reduce the generation of color shift caused by the fact that an average speed of the belt member is different from the average speed during the writing position correcting process due to the switching of the control method between the constant belt speed control and the constant motor speed control.